While performing laser scanning imaging through an optical fiber bundle, the raw output is a collection of samples of the signal directly observed by the detector during the scanning.
As described in U.S. Pat. No. 7,903,848, a raw image can first be reconstructed from this raw output. In this raw image, several pixels can be associated with each given fiber. Each of those pixels associated with a given fiber provide a different measurement of the signal returned by the same spatial volume of the tissue, facing the fiber at its distal end. This returned signal can either arise from fluorescence, back-scattering, or any other radiation resulting from the interaction between the incident˜light and the tissue.
Those various measurements differ because the exact amount of excitation light injected in the fiber during scanning actually depends on the spatial position of the laser spot with respect to the fiber. Furthermore, if the laser spot overlaps with several fibers, the corresponding measurement may compound the returned signal collected by these fibers. From the raw data values, an estimate of the returned signal possibly associated with the spatial volume facing the fiber can typically be made by a somehow ad hoc method as in U.S. Pat. No. 5,878,159 or with a rigorous statistical model as in U.S. Pat. No. 7,903,848. This resulting estimate is then the measurement that one can associate with the given fiber.
This being done, each fiber can conceptually be seen as acting as a single detector, or CCD pixel. But this imaginary CCD differs from common CCD's, at least for four reasons:                a. The fiber bundle is not made of regularly spaced fibers. This imaginary CCD is not square grid;        b. The amount of light injected, transmitted and then collected is also specific to each fiber partly because fiber diameters are not all the same and proximal system optics have aberrations. As a consequence, the pixels of this imaginary CCD usually have, at least, different sensitivities;        c. Some optical scattering effects (Rayleigh, Fresnel, Raman, etc.) may occur at the interfaces or within the fibers, and some signal, either linear or non-linear, may also appear because of it. For those reasons, the returned signal collected from a fiber is not null even if there is no object signal directly produced by the tissue. Also, this background value is different for each fiber; and        d. In addition, those above three phenomena may vary with time. For example, in the context of fluorescence imaging, if the fibers present some auto-fluorescence, its intensity will generally decrease due to photo-bleaching.        
To summarize, after the raw-pixel-to fiber association, each fiber can be seen as a mono-pixel detector with a specific transfer function (e.g. specific offset and specific gain or sensitivity). The imaging system is thus irregular in that the transfer functions are time-varying and non-uniform, i.e. pixel-dependent, and in that the pixels are irregularly located in the spatial domain. In the sequel, for clarity purposes, we refer to these conceptual detectors as fibers, but it should be clear that the discussion need not be restricted to actual physical fibers.
U.S. Pat. Nos. 7,903,848 and 5,878,159 propose different means of processing data coming from detectors with non-uniform fiber transfer functions. U.S. Pat. No. 5,878,159 only discloses an invention in which the fiber transfer functions are simple linear functions (the returned fiber signal is modeled as the multiplication of the object signal by a fiber-dependent gain constant). It does therefore not address the case in which the background signal may not be null when the object signal is null. U.S. Pat. No. 7,903,848 discloses an image processing method that compensates for affine fiber transfer functions. This may be achieved by acquiring two images, one in an homogeneous background medium (e.g. air or water) and one in an homogeneous medium having a strong object signal (e.g. fluorescent solution in case of a fluorescent microscope). The affine fiber transfer functions (i.e. the offsets and gains) are then deduced from these two images. It should be noted that, even though, for clarity purposes, we refer to these reference images as being acquired in homogeneous media, in other contexts, they may simply correspond to homogeneous object signal (e.g. flat field images).
Fiber-based imaging systems are, for example, commonly used to perform endomicroscopy, i.e. endoscopy at a microscopic level. In such a clinical context, acquiring the two images required by U.S. Pat. No. 7,903,848 before each use of the imaging system, i.e. each endomicroscopic procedure, is problematic. Indeed, putting the distal end of the fiber bundle into a homogeneous medium might both, compromise the cleanliness or asepsis of the fiber that may come in contact with the patient, and, burden the procedure for the physician. It should also be clear that in other use cases, finding the proper homogeneous medium might not be trivial. To address this problem but still be able to compensate for affine fiber transfer functions, PCT Pat. Application No. PCTIB2009008012 disclose an invention in which only the background image is acquired, typically in the air, prior to the procedure. This background image is then automatically mapped to data (e.g. background and strong object signal images) that has been acquired previous to the procedure (e.g. during manufacturing) and that is stored together with the imaging system (e.g. on a control computer). From the on-site background image and the mapped previous data, PCT Pat. Application No. PCTIB2009008012 deduces the gains and offsets of the affine fiber transfer functions. The imaging system typically comprises a perduring unit (e.g. laser scanning) and a disposable accessory (e.g. fiber bundle). In the approach of PCT Pat. Application No. PCTIB2009008012, typically only the disposable accessory will be available during manufacturing to acquire the data needed for the on-site image processing, while the perduring unit will remain at the end-user site. Also, it should be apparent that any data acquired during manufacturing will remain constant over time even though the characteristics of the imaging system may vary on the long-term, for example due to aging or environmental characteristics. Hence, the data can only be a biased approximation of what would have been acquired with the complete final imaging system at the time of the image acquisition. This will result in a biased approximation of the fiber transfer functions and thus will lead to a sub-optimal image quality. Also, this solution requires that the disposable accessory be sent to the end-user with the data that corresponds to it. The data needs to be accessed by the image processing method and this will typically require a burdensome installation procedure prior to the first use of the disposable accessory.
Besides the already listed defects of these previous approaches, they all share other common shortcomings. Because the reference images (background and strong object signal) are typically not acquired on a medium having the exact same physical properties (e.g. numerical aperture) as the final object of interest (whose properties are often unknown), the estimated fiber transfer functions will, in any case, only be biased approximations of the fiber transfer functions seen on the object. In other words, these fiber transfer functions typically not only depend on the perduring unit and disposable accessory but depend on the complete imaging setup, imaged medium included.
Furthermore, none of these previous approaches can cope with time-varying fiber transfer functions. As mentioned previously, such scenario is however at least typical in case of fibers that show an auto-fluorescence background signal. Offset values due to fibers auto-fluorescence typically decrease with time, and in a different amount from one fiber to the other, which makes any offset measurement performed before use poorly relevant. Not only does it not reflect the average background level observed in real time during the procedure, but the difference in variation of this background from one fiber to the other affects the image quality and eventually appears as a veil on the image, i.e. a static pattern. Proper pre-illumination of the fibers right before the use of the imaging system may lead to stabilization of the auto-fluorescence and may thus be considered as a potential solution. However this imposes strong constraints to the end-user and does not generalize to other time-varying physical phenomena.
Prior art shows that the problem of non-uniform image detectors arises in other fields and that it has been addressed with different methods as shown for example in Weiss ICCV 2001 or Kuhn et al. Astronomical Society of the Pacific 1991. Accordingly, a need exists for a system and method that would allow for the processing of the data provided by such detectors.